This invention relates to light energy amplification and/or generation for select spectral bands used in optical waveguide communications systems. More particularly, it concerns a method and apparatus having utility both for generation signal carrier bandwidths of light energy introduced to an optical waveguide and for amplifying select bandwidths of such energy propagated in an optical waveguide.
Commonly assigned U.S. Pat. No. 4,342,499 issued Aug. 3, 1982 to John W. Hicks, Jr. discloses the use of tuned, dispersive lateral coupling of two single-mode fiber waveguides by which wavelengths as narrow as 1/1000 of the central wavelength may be selectively filtered. With central wavelengths of about 1 micron, therefore, line width of approximating 10 Angstroms can be filtered from one to the other of the fibers using the dispersive lateral coupling approach to wavelength filtering.
In a commonly assigned, co-pending application Ser. No. 625,543, now U.S, Pat, No. 4,720,160 filed June 28, 1984 by John W. Hicks, Jr., and incorporated herein by reference, a resonant cavity filter system is disclosed for use in optical fiber communication systems by which a line width in the range of 0.1 Angstrom to 0.01 Angstrom may be filtered from a base wavelength again in the 1 micron region of the electromagnetic spectrum. In this latter system, either linear or loop-form optical waveguides are used as resonant cavities tuned to a specific narrow spectral line and laterally coupled between a trunk line, in which light energy at the base wavelength is propagated, preferably as a single mode, and a branch line to which the filtered line width is passed as an information or signal carrying channel.
The major objective of narrow line-width filtering systems of these general types is to enlarge the number of information-bearing channels that can be carried by a single optical waveguide or trunk. The ability to filter line widths as narrow as 0.01 Angstrom, for example, makes available 1 million channels on a single transmission fiber without need for time domain multiplexing, at least on a mathematical or theoretical basis. Obviously, an operating optical waveguide system capable of carrying a number of channels several orders of magnitude lower than the theoretical number of available channels remains very attractive, given the present state of the communications art.
In pursuing the above-noted filtering system of optical fiber waveguides, limitations are encountered as a result of losses in light energy at successive taps of narrow line width channels from a common trunk. Thus, the resonant cavity approach to line width filtering, though offering great potential for directing an extremely narrow line width onto one of several branch lines of a single trunk communication system, is restricted by the light energy loss accumulating in successive taps from a trunk. In this regard, it should first be noted that each tap not only removes the desired wavelength, but also side orders thereof. Conseguently, each successive tap reduces the transmission energy of not merely its own desired wavelength, but of a plurality of wavelengths. Reducing such losses by limiting the amount of energy extracted at each tap is, in turn, restricted by the requirement that a minimum amount of energy at the desired wavelength (e.g., more than 5% of the transmission energy) should be utimately delivered to the branch line for adequate propagation therein.
Another problem in pursuing the potential information carrying capacity of optical waveguides is that present optical waveguide fibers of fused silica with one or the other of the core and cladding doped with material to attain a lower index of refraction in the cladding than in the core, while demonstrating great potential for propagating single-mode light energy at very low losses, are limited to particular regions of the optical spectrum. For example, fused silica is particularly transparent to wavelengths slightly higher than 1 micron, specifically at approximately 1.34 microns. Laser sources capable of developing relatively noise-free signal carrier waves and of introducing wavelengths within this spectral range to an optical waveguide fiber are expensive at least at the present time. While relatively inexpensive laser diodes are available, a combination of low-cost, noise-free operation and operation in the optimum spectral range is expensive to attain given the present state of the laser diode art.
There is an acute need in the optical communications field, therefore, for an effective approach both to the development of noise-free signal carrier waves capable of efficient propagation in optical waveguides and to the amplification of light energy propagated in optical waveguides, particularly light energy at those wavelengths which are tapped or filtered from bandwidths propagated in an optical waveguide trunk, for example.